Daniplestim (SC-55494) is a genetically engineered Il-3 receptor agonist with enhanced multilineage hematopoietic growth factor activity relative to recobinant human interleukin-3 (rhIL-3). In this trial, the sfety and ability of daniplestim coadministered with G-CSF to mobilize peripheral blood stem cells (PBSC) will be compared with G-CSF in breast cancer patients scheduled to receive high dose chemotherapy (HDCT) followed by PBSC infusion. Hematopoietic effects, immune response and tumor cell mobilization will also be analyzed.